Using anti-Nk 1.1 serum, the alloantiserum specific for murine Natural Killer (Nk) cells, we followed the ontogenetic development of Nk-1+ cells in fetal thymus, liver and spleen. A transientpopulation of Nk-1+ cells in fetal thymus was observed on day 14 but not on day 16 of gestation.On day 16 of gestation Nk-1+ cells were detected only in liver and spleen. The proportion of Nk-1+ cells in spleen remained high (20-30%) at birth and persisted until 2 to 3 weeks of age. The Nk-1+ cells in "baby" (1-to-2-week-old) spleen bound to YAC cells but failed to lyse them in51Cr release assays. Upon induction with interferon (IF) the proportion of Nk-1+ cells increased but the lytic activity remained low, suggesting that the "baby" Nk-1+ cells are immaturein lytic function. In adult mice (12-to-14-months-old) Nk-1+ cells were also detectable, but NK activities were lower compared to the young adult (6-to-8-weeks-old) mice. The Nk-1+ cells of old mice were readily induced by IF to exhibit NK activity and the induced NK cells were Nk-1+. We have thus defined Nk-1.1 antigen as an early hemopoietic differentiation antigen. Splenic Nk-1- cells could be induced by IF to become Nk-1+ cells, which could be either inactive or active in NK assays, dependent on the age of the mice.